Be prepared for massive battles in every room-from the beginning where you start till the very end. Ammo is plentiful throughout this level, so don't worry about unloading your strongest weapons at the adversaries you'll face. When you start, you'll be greeted with a massive horde of imps. Use the rocket launcher to blow them all away, or if you don't have that, use the shotgun nearby. Continue killing them until you reach the end, where you'll be forced to fight through some Hell Knights. Move forward after eliminating them. Back in the Red Key room, head to the left teleporter.